Contact driver circuit



A ril 10, 1962 1. G. AKMENKALNS 3,029,351

CONTACT DRIVER CIRCUIT Filed Dec. 19, 1958 1 11 M .1 v.ou1 11 15 15 FIG.1

11 14 I l j v. ou1 15 R I M I+VZ IM/[NTOR IVARS G. AKMENKALNS AGENT pram" United States Paten 3,h29,35l Patented Apr. 10, 1962 ice This invention relates to a contact driver circuit and more particularly to a circuit which utilizes a constant current generator to drive the variable resistance of mechanical contacts.

The problem of contact resistance has always existed when driving signals through mechanical contacts. This resistance can increase its magnitude by several orders through the formation of an insulating film on the c011- tact surfaces. Such a film can be punctured and a normal contact re-established if a temporary break-down voltage is applied across the contacts and in the past a separate source of break-down voltage had to be provided along with suitable switching means for connecting it in circuit with the contacts upon the formation of an insulating film.

The present invention overcomes the need for a separate brake-clown voltage and suitable switching means by employing a constant current generator to drive the variable resistance of the mechanical contacts. In the present invention, a constant current source is used to drive a grounded base transistor which supplies current through mechanical contacts to a resistive load. The resistance of the contacts and the load in effect form a voltage divider, and the grounded base transistor will, within its range of operation, pump a constant current through the mechanical contacts and through the load, thereby providing a constant voltage output. When an insulating film forms on the contact, the resistance of the contacts becomes large with respect to the load resistance, and the collector voltage of the grounded base transistor approaches the saturation point. The circuit is so arranged that the voltage across the contact points will cause the insulating film to break down before the transistor saturates. The spark formed at breakdown disintegrates the film and reforms a highly conducting metallic contact between the switch contacts. With this improved arrangement, a constant current is continuously maintained.

Accordingly, the primary object of the present invention is the provision of a driver circuit which utilizes a constant current generator to drive the variable resistance of mechanical contacts.

A further object of the present invention is to provide a contact driver circuit which includes means for automatically developing a film break-down voltage when an insulating film forms on the contacts.

A still further object of the present invention is to provide a constant current generator and transistor for supplying low power to drive contacts, the full voltage of the generator being made available at the. transistor to reak down any insulator film that may form on the contacts.

Other objects of the invention will be pointed out in the following description and claims and illustrated in the accompanying drawings, which disclose, by Way of examples, the principle of the invention and the best mode, which has been contemplated, of applying that principle.

In the drawings:

FIG. 1 is a circuit diagram of a constant current contact driver in accordance with the present invention.

FIG. 2 is a circuit diagram of a constant current contact driver illustrating another embodiment of the present invention. 7

Referring to the detailed circuit in FIG. 1, there is shown a constant current generator 10 which supplies constant current to the emitter electrode 11 of a grounded base junction type transistor 12, transistor 12 also ineluding a base electrode 13 and acollector electrode 14. Connected in series between hte collector electrode 14 and a voltage output terminal 15 is a resistance R which represents the variable resistance of a pair of mechanical contacts. Connected in series with the contact resistance R is a load resistor R shown connected to a source of positive voltage V, through a terminal 16, the resistances R and R in effect, forming a. voltage divider. Under normal operating conditions, the contact resistance R will be much lower than the load resistance R It is a Well-known fact that the grounded base amplifier 12 will deliver a constant output current in response to a constant input current, and this property permits the amplifier to adjust its output voltage to such a magnitude as to permit constant output current into the load. The practical voltage limit is reached when the load drives the transistor into saturation.

If a contact resistance is developed due to the formation of an insulatingfilm on the contacts, the collector or output voltage of the transistor driver will increase with practically the entire voltage appearing across the contact resistance R, because of the relative magnitude of R with respect to the load resistance R;,. This voltage creates a spark which will break down the insulator-semiconductor film of the contact to re-establish a metallic contact and maintain constant current flow.

If the required break-down voltage is more than the return voltage V, at terminal 16, the circuit embodiment shown in FIG. 2 may be used. This embodiment is similar to the circuit of FIG. 1, with similar components bearing identical reference notations, the essential difference residing in the addition of a branch circuit connected to the collector circuit of the transistor. The branch circuit comprises an inductor L and a'resistor R in a series circuit returned to a voltage V at terminal 1",, which is more positive than the voltage V Under normal operating conditions, the branch consisting of resistances R and R has a much lower total risistance than the resistance in the other branch comprising the inductor L and resistance R. Most of the current I will flow in the R and R branches, and a small voltage disturbance Will be seen at the collector 14 of the grounded base amplifier. However, when an insulating film is formed on the contacts, the R and R branches will have a total resistance greater than the L and R branches, and most of the current will flow in the latter branch. A voltage more positive than the voltage V, Will appear at the collector of the amplifier as the current is reduced due to the voltage generated across the inductor L, and this voltage will break down the insulating film to establish a normal contact. 7

It will be noted that a significant advantage of the present driver circuit resides in the fact that while low power is used to drive the load, the full voltage or" the constant current generator is available to break down the insulator-semiconductor film of the contacts in order to establish a contact.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art, without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. A contact driver circuit comprising, a voltage source,

a voltage divider network connected to said voltage source and including a contact resistance formed by mechanical contacts, and a driver circuit comprising a constant current driver and an amplifier for supplying constant current to said network, the formation of an insulating film on said contacts increasing the contact resistance thereby causing a driver output voltage across said contact resistance which is sufficient to break down said insulating film.

2. A contact driver circuit comprising,a voltage source, a voltage divider network connected to said voltage source and including a contact resistance formed by mechanical contacts, a driver circuit comprising a constant current driver and an amplifier for supplying constant current to said network, a second voltage source, and a branch circuit including an inductor connected between said amplifier and said second voltage source, the formation of an insulating film on said contacts increasing the contact resistance whereby the current flows in said branch circuit and the resulting voltage generated by said inductor upon reduction of drive current appears across said contact resistance to break down said insulating film.

3. A contact driver circuit comprising, a transistor amplifier having emitter, base and collector electrodes, mechanical contacts forming a contact resistance in circuit with said collector electrode, a load circuit cornprising a voltage source and a load resistor in circuit with said contact resistance, and a constant current generator connected to said emitter electrode whereby said transistor provides current through said contact resistance and said load resistor, the formation of an insulating film on said contacts increasing the contact resistance and effecting a voltage increase at said collector electrode sufiicient to break down said insulating film.

4. A contact driver circuit as in claim 3 wherein said transistor has a grounded base configuration to provide a constant current output in responseto said constant current generator input.

5. A contact driver circuit as in claim 3 wherein the contact resistance is much lower under normal operating conditions than the load resistance.

6. A contact driver circuit comprising, a transistor amplifier having emitter, base and collector electrodes, mechanical contacts forming a contact resistance in circuit with said collector electrode, a load circuit comprising a voltage source and a load risistor in circuit with said contact resistance, a branch circuit comprising in series a secon voltage source, a resistor and an inductor, said branch circuit being connected to said collector electrode and in parallel with said contact resistance, and a constant current generator connected to said emitter electrode whereby said transistor provides current through said contact resistance and said load resistance, the for- 'mation of an insulating film on said contacts increasing References Cited in the file of this patent UNITED STATES PATENTS 2,183,838 Hornickel Dec. 19, 1939 Mortimer Aug. 27, 1957 

